For monitoring and controlling light water reactors, reactivity coefficients are used. As a major reactivity coefficient, the moderator temperature coefficient is used for a pressurized water reactor, and the void reactivity coefficient is used for a boiling water reactor.
In a currently implemented technique, to measure a moderator temperature coefficient of a pressurized water reactor, the following operations are performed for changing states of the nuclear power plant:
(1) By inserting control rods, the core power and the coolant temperature are decreased.
(2) The boron concentration in a coolant is controlled to be changed, and the control rods are recovered to their original positions. At this time, an added reactivity is recognized from an actual measured value of a change in boron concentration.
(3) A temperature coefficient is obtained from a change in coolant temperature and the added reactivity.
Also, as another technique, there is a known technique in which, without changing physical states of the plant, time series signals of a coolant temperature and a neutron flux are Fourier-transformed, and from the correlation between them, a moderator temperature coefficient is obtained.
As such the technique, Japanese Patent No. 2895101 discloses a method for measuring a moderator temperature coefficient of a nuclear reactor in operation. In this method, a temperature coefficient is extracted from noise—including signals of reactivity and a coolant temperature by the following manner. First, pieces of time series data on the reactivity and the coolant temperature are collected, and then respectively Fourier-transformed. Subsequently, a cross power spectral density function between the reactivity and the coolant temperature, and the auto power spectral density function of the coolant temperature are calculated. From the ratio between them, the moderator temperature coefficient is calculated.
Also, Japanese Laid-Open Patent Application JP-P2004-125444A discloses a technique in which a decay ratio is used as a core stability index to monitor a degree of stability of a nuclear reactor. The decay ratio is calculated by a singular value decomposition method.